Although radioimmunotherapy with beta emitting radionuclides has demonstrated significant anti-cancer activity, it is severely limited by the long range of the beta particles. As a consequence, except in the radio- sensitive lymphoid cancers, major responses can not be achieved easily without dose limiting myelosuppression. In contrast, targeted alpha particle therapy with Bismuth-213 allows selective killing of single cells and small clusters of cells, but may not be effective in debulking large tumors and is also limited by its short half-life. Over the last 5 years, we have developed human therapeutic model systems for studying both alpha and beta radioimmunotherapy ranging from in vitro analyses and animal models to human clinical studies. We hypothesize that by understanding the geometry, cellular metabolism and catabolism, radiobiology and nuclear chemistry of these radioconstructs in these systems, one can design strategies to take full advantage of their unique and highly active features, while reducing their dose limiting characteristics. AIMS: 1) Explore rational combination of beta and alpha RIT. 2) Investigate in vivo alpha generators with multiple daughters, especially with regard to metabolism. 3) Develop new generator chemistry and reconstruct product purification methods. 4) Elucidate the role of targeted cell antigen density and radioconstruct specific activity in alpha RIT. Each of these issues will be addressed first in models in vitro and in vivo; next, to as great a degree as possible, these evaluations will be translated into human clinical trials.